Study of nonlinear mode coupling during neoclassical tearing modes using bispectrum analysis
It is sometimes observed that several neoclassical tearing modes (NTMs) are present at the same time. This can happen both at the mode onset of a new NTM or during the nonlinear saturated phase of a given mode. In order to analyse whether nonlinear mode coupling plays any role before or after the onset of NTMs, we use a data analysis tool known as the bispectral technique. This technique has been used for identifying the nonlinear three wave interactions in turbulent plasmas. We have used this technique for the study of NTMs in the tokamak a configuration variable (TCV) and in the Joint European Torus (JET). The analysis during the saturated phase of a 3/2 NTM in a JET discharge suggests a strong coupling between the 2, 3 and 5 modes. This coupling could be responsible for the observed stabilization effect of the 4/3, mode on the 3/2 mode (Sauter O et al 2002 Plasma Phys. Control. Fusion 44 1999). It also demonstrates that this analysis tool can be used to identify high m/n modes, beyond the capabilities of the standard method using poloidal or toroidal probe arrays. As this technique allows a more accurate evaluation of the existence of nonlinear mode coupling, it has been used to study and confirm that no significant nonlinear coupling occurred to trigger two specific NTM onset cases in TCV and JET, where a classical tearing mode and a crash after a long sawtooth period are responsible for the triggering of the necessary seed islands, respectively.